Enhanced diaphragm for pressure sensing system and method

ABSTRACT

An enhanced pressure sensing system and method use an external diaphragm to address issues involved with accurate and prolonged measurement of fluid pressure, such as of blood flowing in a vascular structure. Some external diaphragms include a metallized layer or other highly impermeable layer to furnish a high degree of seal at least near to hermetic grade. As temperature of the intermediary fluid changes, the external diaphragm is able to move in a direction that minimizes differential pressure across the external diaphragm over an operational temperature range thereby reducing pressure change of the intermediary fluid due to change in temperature of the intermediary fluid. Relatively smooth hydrodynamic surfaces can be used as well as a bi-layer construction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to pressure sensor systems.

2. Description of the Related Art

A miniature pressure sensor system can include a housing with anexternal diaphragm that directly contacts a sampled fluid, a sensor(enclosed by the housing) that is not in direct contact with the sampledfluid, and an internal intermediary fluid (contained within the housing)that contacts the external diaphragm and also contacts the sensor toallow the sensor to measure pressure of the sampled fluid. In order toaccurately transfer pressure from the sampled fluid to the sensor, amongother things, the external diaphragm must be as compliant with thesampled fluid as possible. Unfortunately, temperature changes can causethe intermediary fluid to expand or contract at a different rate thanpertinent components of the housing thereby inducing pressure changes inthe intermediary fluid unrelated to pressure status of the sampled fluidand consequently, affecting accuracy of pressure measurements of thesampled fluid.

Versions of microelectromechanical systems (MEMS) can determine pressurelevels of a fluid being measured and can be especially useful forminiature pressure sensor systems due to their small size. MEMS pressuresensor dies typically have a MEMS diaphragm fabricated to be integratedin the MEMS die and are typically positioned to directly contact thefluid being measured. In some applications of miniature pressure sensorsystems, a MEMS diaphragm may not be compatible with the particularenvironment and/or the fluid being measured such as intraluminal fluidsfound in biological environments like with blood flows in vascularstructures.

In other cases the MEMS pressure sensor die may be part of othercomponents sharing a common package. These situations can reduce thedesirability of the MEMS diaphragm directly contacting the fluid beingmeasured. In such cases the MEMS diaphragm may be enclosed inside of ahousing having an external diaphragm and a internal intermediary fluidtherebetween. The external fluid is able to contact the sampled fluid tobe measured and the intermediary fluid transfers pressure experienced bythe external diaphragm on to the MEMS diaphragm.

Selection of conventional diaphragms used in non-miniature pressuresensor systems for use as an external diaphragm in miniature pressuresensor systems involving MEMS can be problematic. For instance,highly-stable conventional pressure sensor systems for use in industrialapplications are typically quite large (>15 mm in diameter), havinglarge-diameter, thin metal corrugated diaphragms. Further, theseindustrial applications typically involve a wide range of pressures,which allows such large sized industrial diaphragms to impart a verysmall error in pressure measurement relative to the range of pressuresbeing measured.

In applications involving miniature pressure sensor systems, such aswith vascular structures, where the size of a pressure sensor system isrelatively small and where measured pressures have small ranges and/orvalues, use of conventional diaphragms for an external diaphragm raisescaling issues when going from the larger scale applications to thesmaller scale applications. For instance, in some smaller scaleapplications, such as biomedical applications in general and involvingpulmonary artery pressure measurement in particular, a normal pressurerange may be only from 0 Torr to 20 Torr on a gage basis.

In some of these cases (such as in an implanted medical device), anatmospheric reference pressure is not available, which requires pressuremeasurement using an absolute pressure range from 760 to 780 Torrabsolute that leaves little room for error on a percentage basis andcauses a high demand for accuracy and stability over both time andtemperature. For example, typically, medical diagnosis under suchconditions requires a measurement to within 1 Torr, or 0.13% on anabsolute scale.

Some conventional miniature pressure sensor systems are filled withsilicone or fluorosilicone gel coatings and use no diaphragm. Theoperational life spans of the coatings tend to be short due to thecoatings so are not appropriate where sensor elements require long-termprotection from corrosive body fluids. In addition, these conventionalcoatings expose sample intravascular blood to surfaces that are notfully hemocompatible, such as found with the silicone or fluorosiliconegel materials used for the coatings, which can lead to thrombus (clot)formation within a vascular structure hosting such a miniature pressuresensor system.

In addition, external diaphragm surface geometry is of importance insuch cases when a miniature sensor system is to be placed in flowingfluids such as flowing blood. An undesirable geometry for fluid flowapplications includes rough, non-hydrodynamic surfaces that can induceshearing motion within a blood flow thereby introducing a potentialactivation mechanism of a coagulation cascade, leading to thrombus(clot) formation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a perspective view of a pressure sensing system with the coverremoved.

FIG. 2 is an exploded side elevational view of the pressure sensingsystem of FIG. 1.

FIG. 3 is a top plan view of the pressure sensing system of FIG. 1without the cover.

FIG. 4 is a sectional view of the pressure sensing system taken alongthe 4-4 line of FIG. 3.

FIG. 5 is a top plan view of a MEMS pressure sensing die of the pressuresensing system of FIG. 1.

FIG. 6 is a top plan view of a sensor containment cover of the pressuresensing system of FIG. 1.

FIG. 7 is a sectional view of the sensor containment cover taken alongthe 7-7 line of FIG. 6.

FIG. 8A is a top plan view of an enhanced external diaphragm of thepressure sensing system.

FIG. 8B is a sectional view of the external diaphragm of FIG. 8A.

FIG. 9A is a top plan view of a corrugated version of the externaldiaphragm of FIG. 8A.

FIG. 9B is a sectional view of the corrugated external diaphragm of FIG.9A.

FIG. 10A is a top plan view of the corrugated external diaphragm of FIG.9A with a filler material.

FIG. 10B is a sectional view of the corrugated external diaphragm withthe filler material of FIG. 10A.

FIG. 11A is a top plan view of corrugated external diaphragm of FIG. 10Awith an added external layer.

FIG. 11B is a sectional view of the corrugated external diaphragm withan added external layer of FIG. 11A.

DETAILED DESCRIPTION OF THE INVENTION

An enhanced pressure sensing system and method of miniature size, suchas for implantation in a biological structure such as a lumen, vessel,or other tubular structure, is discussed herein that addresses issuesinvolved with accurate and prolonged measurement of fluid pressure, suchas of blood flowing in a vascular structure. The issues includeproviding at least close to hermetic isolation between a sampled fluidand a pressure sensor, providing high compliance levels of the miniaturepressure sensor system with the host environment (such as a vascularstructure containing flowing blood), and maintaining accuratecorrespondence between internal intermediary fluid pressure and sampledfluid pressure, so that sampled fluid pressure state is accuratelytransferred to the internal sensor.

Implementations of external diaphragms are incorporated into theenhanced pressure sensing system to address such issues. For instance,some implementations of the external diaphragm include a metallizedlayer or other highly impermeable layer to furnish a high degree of sealat least near to hermetic grade. Implementations can include variousstructural considerations of the external diaphragm that allow for aregulated adjustment of volume containing the internal intermediaryfluid to allow for expansion and contraction of the intermediary fluiddue to temperature change to diminish changes in pressure of theintermediary fluid due to such temperature change. As temperature of theintermediary fluid changes, the external diaphragm is able to move in adirection that minimizes differential pressure across the externaldiaphragm over an operational temperature range (such as 0 to 100degrees Celsius or 33 to 43 degrees Celsius) thereby reducing pressurechange of the intermediary fluid due to change in temperature of theintermediary fluid. Implementations can also include structuralconsiderations of the external diaphragm, which would allow for arelatively smooth hydrodynamic surface to reduce possibilities ofinducing shearing motion in sampled fluid flow.

Implementations of the external diaphragm include a bi-layerconstruction to provide hermetic-like sealing and an ability to expandand contract to compensate for temperature change of the intermediaryfluid. The two layers of the bi-layer external diaphragm can have asimilar thickness and can be made from materials of comparable elasticmodulus but maximally different coefficients of thermal expansion (CTE).Alternately, if the elastic moduli are different, the thicknesses of thematerials of the two layers of the bi-layer external diaphragm can bechosen so that the stiffness of each layer is approximately equal toincrease deflection of the diaphragm with temperature. Examples are twometal layers (where the thicknesses would be comparable) or a ceramiccoated with a polymer (where the polymer layer would be much thickerthan the ceramic).

In some implementations, one of the layers, such as the layer contactingthe intermediary fluid, can be made from an impermeable material thatallows the housing of the pressure sensor system to be hermetic ornear-hermetic. Implementations include one of the layers of the bi-layerexternal diaphragm (such as the external layer in contact with thesampled fluid) to have a hemocompatible outer surface material to reducerisk of thrombus formation.

In some implementations, the thickness of the first layer and thethickness of the second layer are sized according to in part thedifference between the coefficient of thermal expansion of theintermediary fluid and the coefficient of thermal expansion of thehousing portions defining a chamber containing the intermediary fluidand also according to in part the difference between the coefficient ofthermal expansion of the first layer and second coefficient of thermalexpansion of the second layer to allow for expansion of the chamber whentemperature of the intermediary fluid and the housing portions isincreased for conditions including over an operational temperature rangeof the pressure measuring system.

An exemplary version of the pressure sensing system is depicted hereinas a representative example of how the bi-layer external diaphragm isused with a MEMS diaphragm positioned with a MEMS pressure sensor die ina housing to indirectly sample pressure state of a fluid being measured.The external diaphragm is used to make direct contact with the fluidbeing measured. Pressure state of the fluid being measured istransferred from the external diaphragm in direct contact through anelectrically insulating intermediary fluid to the MEMS diaphragm therebyallowing the MEMS pressure sensor die to indirectly sample pressurestate of the fluid being measured. Electrically conductive supportmembers and electrically conductive solid vias are used to electricallycouple circuitry outside the housing.

An exemplary pressure sensing system 100 is depicted in FIG. 1 as havinga pressure sensing assembly 102 enclosed by a component package 103having a package base 104 and a cover 106 shaped to sealably couple withthe package base. In some implementations, the package base 104 can bemade from a substrate material. The pressure sensing assembly 102 isintegrally jointed with the package base 104 with a portion of thepackage base serving as a wall for the pressure sensing assembly 102.

The pressure sensing assembly 102 includes a housing 107 with a housingcover 108 having an exterior surface 108 a, an interior surface 108 b,and conductive solid vias 110 extending therebetween (better shown inFIG. 4). The housing 107 can be formed from ceramic and attached to thepackage base 104 with epoxy, silicone, brazing, or other attachmentmeans. Alternatively, the housing 107 can be formed from metal andattached to the package base 104 with epoxy, silicone, brazing, laserwelding, or other attachment means. As depicted, the housing cover 108can be a hybrid printed circuit board formed from glass, ceramic, orother mechanically stable material compatible with technology involvingprinted circuitry. The housing cover 108 can be sealed to the housing107 such as with epoxy, silicone, or braze. The housing 107 alsoincludes a plughole 112 sized to receive a plug 114.

As shown in FIG. 2, the pressure sensing assembly 102 further includes aMEMS pressure sensor die 116 that is in electrical contact withelectrically conductive support members 118, which in turn are inelectrical contact with the electrically conductive solid vias 110further discussed below. The MEMS pressure sensor die 116 can bedesigned to determine fluid pressure levels either through capacitive orpiezoresistive means. The conductive support members 118 alsomechanically couple the MEMS pressure sensor die 116 to the housingcover 108. In some implementations, portions of solder, such as solderbumps, or other types of bumps such as stud bumps which are typicallymade of gold, are used for the conductive support members 118. Alsoshown in FIG. 2 as included with the pressure sensing assembly 102 is aexternal diaphragm 120 that is positioned to seal a housing aperture 121shown in cross-section in FIG. 4, in that portion of the package base104 that serves as a wall of the pressure sensing assembly. The externaldiaphragm 120 can be attached to the package base 104 by laser welding,epoxy, silicone, electrochemical bonding, electrochemical forming,brazing or other means. The external diaphragm 120 has a first layer 120a to contact a fluid being measured that has a pressure value that isdesired to be known and an oppositely facing second layer 120 b.

As indicated in FIG. 3, a sectional view of the pressure sensingassembly 102 is found in FIG. 4 showing the package base 104 to have aplug aperture 122 communicating with the plughole 112. As is shown, theMEMS pressure sensor die 116 has a MEMS diaphragm 123 in communicationwith a chamber 124 defined in part by the external diaphragm and housingportions including the package base 104, the housing 107, and thehousing cover 108. The chamber 124 contains an intermediary fluid 125,which is used to transfer pressure applied to the first layer 120 a ofthe external diaphragm 120 to the MEMS diaphragm 123. A fill port 126 isalso in communication with the chamber 124 when the plug 114 is removedfrom the plughole 112 to fill the chamber with the intermediary fluid125. The fill port 126 can be machined in the housing 107 and providesaccess to the chamber 124. The plughole 112 can be drilled in thehousing 107 to receive the plug 114, which is used to seal the fill port126 and the chamber 124 after the chamber has been filled with theintermediary fluid. The plug 114 can be made as a stopcock or otherdevice to seal the fill port 126 and the chamber 124 as long as thevolume of the sealed chamber has the same value each time the chamber issealed.

The MEMS pressure sensor die 116 is shown in FIG. 5 to includeelectrical circuitry contacts 128 used for electrical communication withthe MEMS pressure sensor die, such as to output pressure values orstatus information from the MEMS pressure sensor die or to input controlsignals to the MEMS pressure sensor die. The circuitry contacts 128 ofthe MEMS pressure sensor die 116 are aligned with or otherwiseelectrically connected to the conductive solid vias 110 in the housingcover 108, better shown in FIGS. 6 and 7 to conduct the electricalcommunication with the MEMS pressure sensor die outside of the housing107. The conductive solid core vias 110 of the housing cover 108 act assealed electrical feedthrough interconnects between the exterior surface108 a and the interior surface 108 b, which is electrically coupled tothe electrical circuitry contacts 128 of the MEMS pressure sensor die116 through the conductive support members 118. External wires 130 areattached to the external surface 108 a of the conductive vias 110 withsolder, wire bond, conductive epoxy or other electrical bonding method.

In operation, the chamber 124 is filled with an intermediary fluid 125that is electrically insulating, such as a silicone-based fluid such assilicone oil, perfluorocarbon liquid, or other insulator based fluid.The chamber 124 can be vacuum filled with fluid by first placing theentire pressure sensing assembly in a vacuum chamber and evacuating it,and while under vacuum, immersing the assembly 102 in the fluid. As airpressure is reintroduced into the vacuum chamber, it forces the fluid tofill any internal spaces within the pressure sensing assembly, leavingto voids. The plug 114 can then be inserted into the plughole 112 toseal off the chamber 124. Alternately, an additional plughole 112 and anadditional fill port 126 can be used as a vent to introduce pressurizedsilicone oil or other pressurized fluid in the chamber 124. Afterfilling the chamber 124, all of the plugholes 112 each would be sealedwith one of the plugs 114.

External fluid pressure impinging upon the first layer 120 a of theexternal diaphragm 120 sealing the housing aperture 121 of the packagebase 104 is transferred to the intermediary fluid 125 in the chamber 124through the external diaphragm whereby the MEMS diaphragm is affectedand the MEMS pressure sensor die senses a pressure level correspondingto the external fluid pressure.

Now turning to further discuss the external diaphragm 120 in moredetail, the external diaphragm 120 is generally configured to be able tomove freely over the operating pressure range and also over theoperating temperature range so that it does not introduce a significantpressure differential, which would result in an error in the pressurereading. In addition, the external diaphragm 120 must be maintained in alinear, elastic region, as opposed to a plastic region, as it isdeformed, so that its behavior does not change over temperature andpressure cycles, altering the calibration of the sensor system.

The intermediary fluid 125 poses a challenge in that it expandsvolumetrically with increasing temperature at a rate higher than thehousing, resulting in an outward pressure on the external diaphragm 120.The amount of fluid expansion is characterized by the coefficient ofthermal expansion (CTE) for the intermediary fluid 125.

A first implementation of the external diaphragm 120 is shown in FIG. 8Aand FIG. 8B with the first layer 120 a and the second layer 120 b havingdifferent thickness and different thermal expansion coefficients. Thefirst layer 120 a is made from an impermeable material to allow theexternal diaphragm 120 to hermetically seal the intermediary fluid 125.For instance, the first layer 120 a could be a sputtered, evaporated, orplated metal film on the second layer 120 b, which could be a plasticmaterial.

Being made of a plastic material, the second layer 120 b would have amuch higher CTE than the metal material of the first layer 120 a so thatthe external diaphragm 120 would dome outward away from the chamber 124and toward the sampled fluid as the temperature of the diaphragm and theintermediary fluid 125 increased. This thermal expansion would increasethe volume of the chamber 124 and given appropriate sizing of the firstlayer 120 a and the second layer 120 b relative to the dimensions of thechamber, the volume of the chamber could expand at the same rate as thevolumetric expansion of the intermediary fluid 125. In thisimplementation, the exterior surface of the second layer 120 b incontact with the sampled fluid being measured is smooth to minimizethrombus formation.

A second implementation of the external diaphragm 120 is shown in FIG.9A and FIG. 9B in which the first layer 120 a and the second layer 120 bare of a corrugated construction with the first layer and the secondlayer having different thickness and different thermal expansioncoefficients. The corrugation of the first layer 120 a and the secondlayer 120 b increase the compliance of the external diaphragm 120, toreduce differential pressure across the external diaphragm astemperature of the intermediary fluid 125 changes. The first layer 120 ais of an impermeable material to provide hermetic sealing of theintermediary fluid 125. The first layer 120 a can be a sputtered,evaporated, or plated metal film on the second layer 120 b, which can beof a plastic material.

The plastic material of the second layer 120 b can have a much higherCTE than the metal material of the first layer 120 a so that theexternal diaphragm 120 can dome outward as temperature of theintermediary fluid 125 is increased as further described above. Sincethe second implementation of the external diaphragm 120 has a corrugatedexterior surface of the second layer 120 b in contact with the sampledfluid, the second implementation does not minimize thrombus formation tothe same extent as the first implementation of the external diaphragmwith the smooth exterior surface of the second layer 120 b.

A third implementation of the external diaphragm 120 is shown in FIG.10A and FIG. 10B as having a corrugated first layer 120 a and acorrugated second layer 120 b having different thickness and differentthermal expansion coefficients. The first layer 120 a is impermeable toprovide hermetic sealing for the intermediary fluid 125. The first layer120 a can be a sputtered, evaporated, or plated metal film on the secondlayer 120 b, which could be a plastic material. The plastic material ofthe second layer 120 b can have a much higher CTE than the metalmaterial of the first layer 120 a so that the external diaphragm 120 candome outward as temperature of the intermediary fluid 125 as furtherdescribed above.

As shown in FIG. 10B, the outer corrugated surface of the second layer120 b is filled with a hydrogel or a silicone, fluorosilicone,perfluorocarbon gel type first cover material 120 c, which has a lowmodulus. The first cover material 120 c provides a smooth outer surfaceof the external diaphragm that is in contact with the sampled fluid tominimize thrombus formation. The volumetric CTE of hydrogels andsilicone, fluorosilicone, or perfluorocarbon gels for the first covermaterial 120 c is typically 1065-1305 PPM/° C., and thus the linear CTErange is 355-435 PPM/° C. This is substantially higher than solidplastic materials used for the second layer 120 b, which are in therange of 40-250 PPM/° C. (where the higher values correspond to softmaterials such as silicone rubber). Thus, thermal expansion of the firstcover material 120 c can assist in causing the external diaphragm 120 todome outward with increasing temperature of the intermediary fluid 125(although this will be a weak effect, due to the low modulus of thegel).

Choosing a smooth outer surface material for the external diaphragm tocontact the sampled fluid addresses the thrombus formation issue, butuse of a silicone, fluorosilicone, or perfluorocarbon gel for the firstcover material 120 c is not ideal regarding hemocompatibility when thesampled fluid involves blood flow. Chemical mechanisms can also triggera coagulation cascade, and use of silicone, fluorosilicone, orperfluorocarbon gel for the first cover material 120 c can also elicit aforeign body response (leading to a series of events as host body seeksto encapsulate the cover material). Using a hydrogel for the first covermaterial 120 c can minimize this problem, as certain hydrogelformulations are known to be well-tolerated in the bloodstream.

A fourth implementation of the exterior diaphragm 120 is shown in FIG.11A and FIG. 11B that incorporates aspects of the first layer 120 a, thesecond layer 120 b, and the first cover material 120 c of the thirdimplementation of the exterior diaphragm. Further included in the fourthimplementation is a second cover material 120 d, which covers the firstcover material 120 c. The second cover material 120 d can provide asmooth surface and also have hemocompatibility properties, such as foundwith expanded polytetrafluoroethylene (ePTFE). The second cover material120 d of ePTFE can be made with a range of porosities, pore sizes, andsurface characteristics, for instance, having similar characteristics asa vascular graft application, as both are in direct blood contact.

The ePTFE, being a porous Teflon material, has a very low tensilemodulus of about 48.6 MPa, as compared to 552 MPa for solid PTFE and3.45 GPa for polyetheretherketone (PEEK), a rigid plastic. Titanium, bycomparison, is 110 GPa. Thus, the ePTFE layer will be highly compliantand will not interfere with normal pressure sensor operation. Choosing asmooth outer surface material for the second cover material 120 daddresses the thrombus formation issue, and the ePTFE is a good choicefrom a hemocompatibility standpoint.

The pore size of ePTFE in a form that is used in vascular grafts istypically 10 to 30 microns, which is larger than blood cells (˜8microns) or other types of cells found in the blood. Using ePTFE with asmaller pore size (<8 microns) provides a surface that the cells cannotpenetrate. Thus, cells cannot enter into the porous space within thematerial. Thus, the space, if the first cover material 120 c is not usedso that the space between the second layer 120 b and the second covermaterial 120 d is left unfilled at the time of implantation, the spacebecomes filled with fluid constituents of the blood. If the first covermaterial 120 c is used to fill the space between the second layer 120 band the second cover material 120 d, the first cover material could alsobe introduced into the porosity of the ePTFE of the second covermaterial, if so desired, to exclude fluid constituents of the blood fromentering into the pore space of the second cover material. In this case,the ePTFE pore size could be larger than a blood cell, if so desired.Using a first cover material 120 c that readily wets ePTFE and which haslow solubility in the bloodstream, such as a perfluorocarbon-based gelor liquid, offers the added advantage that the material 120 c willlargely remain in place within the ePTFE. It is hypothesized thatfilling the ePTFE porosity with the first cover material may improvesensor longevity in the body, as it would prevent biofouling fromoccurring within the ePTFE over extended periods.

From the foregoing it will be appreciated that, although specificimplementations have been described herein for purposes of illustration,various modifications may be made without deviating from the spirit andscope of the invention.

1. For sealing an aperture, an external diaphragm, the apertureotherwise in fluid communication with a chamber containing anintermediary fluid except for sealing of the aperture by the externaldiaphragm, the intermediary fluid having a pressure level, the chamberbeing defined by housing portions and the external diaphragm, thehousing portions having an overall coefficient of thermal expansion andbeing included in an implantable pressure measuring system to measurepressure of a sampled fluid within a biological structure, the pressuremeasuring system further including a pressure sensor in fluidcommunication with the intermediary fluid, the intermediary fluid havinga coefficient of thermal expansion larger by a difference than theoverall coefficient of thermal expansion of the housing portions, theexternal diaphragm comprising: a first diaphragm portion having a firstcoefficient of thermal expansion, the first diaphragm portion positionedin the external diaphragm to contact the intermediary fluid when theexternal diaphragm is sealing the aperture with the pressure measuringsystem implanted in the biological structure; and a second diaphragmportion having a second coefficient of thermal expansion, the seconddiaphragm portion in juxtaposition with the first diaphragm portion, thefirst diaphragm portion and the second diaphragm portion, shaped, sized,and positioned in the external diaphragm to cause the external diaphragmto expand away from the chamber to increase volume of the chamber whentemperature of the intermediary fluid and the housing portions increasesto substantially compensate for the difference between the coefficientof thermal expansion of the intermediary fluid and the overallcoefficient of thermal expansion of the housing portions and geometricalconfiguration of the housing portions to allow the pressure level of theintermediary fluid to be substantially unaffected by changes intemperature of the intermediary fluid for conditions including over anoperational range of the pressure measuring system.
 2. The method ofclaim 1, wherein the first diaphragm portion and the second diaphragmportion are corrugated, further including a first cover materialpositioned to fill corrugated surfaces of the second diaphragm portion.3. The method of claim 2, wherein the first cover material is one of thefollowing: a hydrogel, a silicone gel, a fluorosilicone gel, and aperfluorocarbon gel.
 4. The method of claim 3, further including asubstantially smooth second cover material positioned adjacent to thefirst cover material.
 5. The method of claim 4, wherein the second covermaterial is substantially expanded polytetrafluoroethylene.
 6. Themethod of claim 5, wherein the porosity of the second cover material isfilled with a gel or a liquid.
 7. The external diaphragm of claim 1wherein the first diaphragm portion is corrugated.
 8. The externaldiaphragm of claim 7 wherein the second diaphragm portion is corrugatedto increase compliance of the external diaphragm to reduce potential ofa pressure differential forming across the external diaphragm as aresult of temperature change of the intermediary fluid.
 9. The method ofclaim 8, wherein the first coefficient of thermal expansion of the firstdiaphragm portion is less than the second coefficient of thermalexpansion of the second diaphragm portion.
 10. The method of claim 1,wherein the second diaphragm portion is positioned in the externaldiaphragm to be in contact with the sampled fluid when the externaldiaphragm is positioned to seal the aperture.
 11. The method of claim10, wherein the second diaphragm portion has a substantially smoothsurface to contact the sampled fluid.
 12. The method of claim 1, whereinthe second diaphragm portion has a thickness greater than the firstdiaphragm portion.
 13. The method of claim 1, wherein the firstdiaphragm portion is substantially impermeable to the intermediary fluidassist in preventing passage of the intermediary fluid from the chamberwhen the external diaphragm is positioned to seal the aperture.
 14. Themethod of claim 1, wherein the first diaphragm portion is coupled to thesecond diaphragm portion by at least one of the following: sputtering,evaporation, and plating.
 15. The method of claim 1, wherein the firstdiaphragm portion is coupled to the second diaphragm portion as a thinmetal film.
 16. The method of claim 1, wherein the second diaphragmportion is a plastic material.
 17. For sealing an aperture, an externaldiaphragm, the aperture in fluid communication with a chamber containingan intermediary fluid having a pressure level, the chamber being definedby housing portions and the external diaphragm, the housing portionshaving an overall coefficient of thermal expansion and being included inan implantable pressure measuring system to measure pressure of asampled fluid within a biological structure, the pressure measuringsystem further including a pressure sensor in fluid communication withthe intermediary fluid, the intermediary fluid having a coefficient ofthermal expansion larger by a first difference than the overallcoefficient of thermal expansion of the housing portions, the externaldiaphragm comprising: a first layer having a first thickness and a firstcoefficient of thermal expansion, the first layer being substantiallyimpermeable and positioned in the external diaphragm to be in contactwith the intermediary fluid when the external diaphragm is positioned toseal the aperture and the chamber contains the intermediary fluid; and asecond layer in juxtaposition with the first layer, the second layerhaving a second coefficient of thermal expansion and a second thickness,the second coefficient of thermal expansion being greater by a seconddifference than the first coefficient of thermal expansion, the firstthickness of the first layer and the second thickness of the secondlayer being sized according to in part the first difference between thecoefficient of thermal expansion of the intermediary fluid and thecoefficient of thermal expansion of the housing portions and the seconddifference between the first coefficient of thermal expansion of thefirst layer and the second coefficient of thermal expansion of thesecond layer to allow for expansion of the chamber when temperature ofthe intermediary fluid and the housing portions is increased forconditions including over an operational temperature range of thepressure measuring system.
 18. The method of claim 17, wherein the firstlayer and the second layer are corrugated, further including a firstcover material positioned to fill corrugated surfaces of the secondlayer.
 19. The method of claim 18, wherein the first cover material isone of the following: a hydrogel, a silicone gel, a fluorosilicone gel,and a perfluorocarbon gel.
 20. The method of claim 19, further includinga substantially smooth second cover material positioned adjacent to thefirst cover material.
 21. The method of claim 20, wherein the secondcover material is substantially expanded polytetrafluoroethylene. 22.The method of claim 21, wherein the porosity of the second covermaterial is filled with a gel or a liquid.
 23. The external diaphragm ofclaim 17 wherein the first layer is corrugated.
 24. The externaldiaphragm of claim 23 wherein the second layer is corrugated to increasecompliance of the external diaphragm to reduce potential of a pressuredifferential forming across the external diaphragm as a result oftemperature change of the intermediary fluid.
 25. The method of claim23, wherein the first coefficient of thermal expansion of the firstlayer is less than the second coefficient of thermal expansion of thesecond layer.
 26. The method of claim 17, wherein the second layer ispositioned in the external diaphragm to be in contact with the sampledfluid when the external diaphragm is positioned to seal the aperture.27. The method of claim 26, wherein the second layer has a substantiallysmooth surface to contact the sampled fluid.
 28. The method of claim 17,wherein the second layer has a thickness greater than the first layer.29. The method of claim 17, wherein the first layer is substantiallyimpermeable to the intermediary fluid is assist in preventing passage ofthe intermediary fluid from the chamber when the external diaphragm ispositioned to seal the aperture.
 30. The method of claim 17, wherein thefirst layer is coupled to the second layer by at least one of thefollowing: sputtering, evaporation, and plating.
 31. The method of claim17, wherein the first layer is coupled to the second layer as a thinmetal film.
 32. The method of claim 17, wherein the second diaphragmportion is a plastic material.
 33. A method for providing an externaldiaphragm for sealing an aperture, the aperture in fluid communicationwith a chamber containing an intermediary fluid having a pressure level,the chamber being defined by housing portions having an overallcoefficient of thermal expansion and being included in an implantablepressure measuring system to measure pressure of a sampled fluid withina biological structure, the pressure measuring system further includinga pressure sensor in fluid communication with the intermediary fluid,the intermediary fluid having a coefficient of thermal expansion largerby a difference than the overall coefficient of thermal expansion of thehousing portions, the method comprising: providing a first diaphragmportion having a first coefficient of thermal expansion, the firstdiaphragm portion positioned in the external diaphragm to contact theintermediary fluid when the external diaphragm is sealing the aperturewith the pressure measuring system implanted in the biologicalstructure; and providing a second diaphragm portion having a secondcoefficient of thermal expansion, the second diaphragm portion injuxtaposition with the first diaphragm portion, shaping, sizing, andpositioning the first diaphragm portion and the second diaphragm portionto cause the external diaphragm to expand away from the chamber toincrease volume of the chamber when temperature of the intermediaryfluid and the housing portions increases and to substantially compensatefor the difference between the coefficient of thermal expansion of theintermediary fluid and the overall coefficient of thermal expansion ofthe housing portions and geometrical configuration of the housingportions to allow the pressure level of the intermediary fluid to besubstantially unaffected by changes in temperature of the intermediaryfluid for conditions including over an operational range of the pressuremeasuring system; and coupling the first diaphragm portion with thesecond diaphragm portion.
 34. The method of claim 33, wherein the firstdiaphragm portion and the second diaphragm portion are corrugated,further including positioning a first cover material to fill corrugatedsurfaces of the second diaphragm portion.
 35. The method of claim 34,wherein the first cover material is one of the following: a hydrogel, asilicone gel, a fluorosilicone gel, and a perfluorocarbon gel.
 36. Themethod of claim 34, further including providing a substantially smoothsecond cover material; and positioning the second cover materialadjacent to the first cover material.
 37. The method of claim 36,wherein the second cover material is substantially expandedpolytetrafluoroethylene.
 38. The method of claim 37, wherein theporosity of the second cover material is filled with a gel or a liquid.39. The external diaphragm of claim 33, further including shaping thesecond diaphragm portion to increase compliance of the externaldiaphragm to reduce potential of a pressure differential forming acrossthe external diaphragm as a result of temperature change of theintermediary fluid.
 40. The method of claim 39, wherein the seconddiaphragm portion is shaped as corrugated.
 41. The method of claim 39,wherein the first coefficient of thermal expansion of the firstdiaphragm portion is lesser than the second coefficient of thermalexpansion of the second diaphragm portion.
 42. The method of claim 33,further including positioning the second diaphragm portion in theexternal diaphragm to be in contact with the sampled fluid when theexternal diaphragm is positioned to seal the aperture.
 43. The method ofclaim 42, wherein the second diaphragm portion has a substantiallysmooth surface to contact the sampled fluid.
 44. The method of claim 33,wherein the second diaphragm portion has a thickness greater than thefirst diaphragm portion.
 45. The method of claim 33, wherein the firstdiaphragm portion is substantially impermeable to the intermediary fluidis assist in preventing passage of the intermediary fluid from thechamber when the external diaphragm is positioned to seal the aperture.46. The method of claim 33, further including coupling the firstdiaphragm portion to the second diaphragm portion by at least one of thefollowing: sputtering, evaporation, and plating.
 47. The method of claim33, further including coupling the first diaphragm portion to the seconddiaphragm portion as a thin metal film.
 48. The method of claim 33,wherein the second diaphragm portion is a plastic material.
 49. A methodfor providing an external diaphragm for sealing an aperture, theaperture in fluid communication with a chamber containing anintermediary fluid having a pressure level, the chamber being defined byhousing portions and the external diaphragm, the housing portions havingan overall coefficient of thermal expansion and being included in animplantable pressure measuring system to measure pressure of a sampledfluid within a biological structure, the pressure measuring systemfurther including a pressure sensor in fluid communication with theintermediary fluid, the intermediary fluid having a coefficient ofthermal expansion larger by a first difference than the overallcoefficient of thermal expansion of the housing portions, the methodcomprising: providing a first layer having a first thickness and a firstcoefficient of thermal expansion, the first layer being substantiallyimpermeable; positioning the first layer in the external diaphragm to bein contact with the intermediary fluid when the external diaphragm ispositioned to seal the aperture and when the chamber contains theintermediary fluid; and positioning a second layer in the externaldiaphragm in juxtaposition with the first layer; positioning the secondlayer in the external diaphragm to be in contact with the sampled fluidwhen the external diaphragm is positioned to seal the aperture and thepressure measuring system is implanted in the biological structure;providing the second layer having a second coefficient of thermalexpansion and a second thickness, the second coefficient of thermalexpansion being greater by a second difference than the firstcoefficient of thermal expansion, sizing the first thickness of thefirst layer and the second thickness of the second layer being accordingto in part the first difference between the coefficient of thermalexpansion of the intermediary fluid and the coefficient of thermalexpansion of the housing portions and the second difference between thefirst coefficient of thermal expansion of the first layer and the secondcoefficient of thermal expansion of the second layer to allow forexpansion of the chamber when temperature of the intermediary fluid andthe housing portions is increased for conditions including over anoperational temperature range of the pressure measuring system.
 50. Themethod of claim 49, wherein the first layer and the second layer arecorrugated, further including providing a first cover material andpositioning the first cover material to fill corrugated surfaces of thesecond layer.
 51. The method of claim 50, wherein the first covermaterial is one of the following: a hydrogel, a silicone gel, afluorosilicone gel, and a perfluorocarbon gel.
 52. The method of claim50, further including providing a substantially smooth second covermaterial and positioning the second cover material adjacent to the firstcover material.
 53. The method of claim 52, wherein the second covermaterial is substantially expanded polytetrafluoroethylene.
 54. Themethod of claim 53, wherein the porosity of the second cover material isfilled with a gel or a liquid.
 55. The external diaphragm of claim 49wherein the first layer is corrugated.
 56. The external diaphragm ofclaim 55, further including shaping the second layer to increasecompliance of the external diaphragm to reduce potential of a pressuredifferential forming across the external diaphragm as a result oftemperature change of the intermediary fluid.
 57. The method of claim56, wherein the second layer is shaped as corrugated.
 58. The method ofclaim 55, wherein the first coefficient of thermal expansion of thefirst layer is lesser than the second coefficient of thermal expansionof the second layer.
 59. The method of claim 49, further includingpositioning the second layer in the external diaphragm to be in contactwith the sampled fluid when the external diaphragm is positioned to sealthe aperture.
 60. The method of claim 59, wherein the second layer has asubstantially smooth surface to contact the sampled fluid.
 61. Themethod of claim 49, wherein the second layer has a thickness greaterthan the first layer.
 62. The method of claim 49, wherein the firstlayer is substantially impermeable to the intermediary fluid is assistin preventing passage of the intermediary fluid from the chamber whenthe external diaphragm is positioned to seal the aperture.
 63. Themethod of claim 49, wherein the first layer is coupled to the secondlayer by at least one of the following: sputtering, evaporation, andplating.
 64. The method of claim 49, wherein the first layer is coupledto the second layer as a thin metal film.
 65. The method of claim 49,wherein the second diaphragm portion is a plastic material.